In the construction of many buildings, a finished ceiling, which is referred to as a drop ceiling, is spaced below a structural floor panel that is constructed of concrete, for example. Light fixtures as well as other items appear below the drop ceiling. The space between the ceiling and the structural floor from which it is suspended serves as a return-air plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications cables including those for computers and alarm systems. It is not uncommon for these plenums to be continuous throughout the length and width of each floor.
When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly throughout an entire story of the building. The fire could travel along the length of cables which are installed in the plenum. Also, smoke can be conveyed through the plenum to adjacent areas and to other stories.
Generally, a non-plenum cable sheath system which encloses a core of insulated copper conductors and which comprises only a conventional plastic jacket does not exhibit acceptable flame spread and smoke evolution properties. As the temperature in such a cable rises, charring of the jacket material begins. Afterwards, conductor insulation inside the jacket begins to decompose and char. If the jacket char retains its integrity, it functions to insulate the core; if not, it is ruptured by the expanding insulation char, exposing the virgin interior of the jacket and insulation to elevated temperatures. The jacket and the insulation begin to pyrolize and emit flammable gases. These gases ignite and, because of air drafts within the plenum, burn beyond the area of flame impingement, propagating flame and evolving smoke.
As a general rule, the National Electrical Code (NEC) requires that power-limited cables in plenums be enclosed in metal conduits. The initial cost of metal conduits for communications cables in plenums is relatively expensive. Also, conduit is relatively inflexible and difficult to maneuver in plenums. Further, care must be taken during installation to guard against possible electrical shock which may be caused by the conduit engaging exposed electrical service wires or equipment. However, the Code permits certain exceptions to this requirement provided that such cables are tested and approved by an authority such as the Underwriters' Laboratories.
The problem of acceptable plenum cable design is complicated somewhat by the trend to the extension of the use of optical fiber transmission media from a loop to building distribution systems. Not only must the optical fibers be protected from transmission degradation, but also they have properties which differ significantly from those of copper conductors and hence require special treatment. Light transmitting optical fibers are mechanically fragile, exhibiting low strain facture under tensile loading and degraded light transmission when bent. The degradation in transmission which results from bending is known as microbending loss. This loss can occur because of coupling between the jacket and the core. This may result because of shrinkage during cooling of the jacket and because of differential thermal contractions when the thermal properties of the jacket material differ significantly from those of the enclosed optical fibers.
A plenum cable which includes a core of copper conductors is shown in U.S. Pat. No. 4,284,842 which issued on Aug. 18, 1981 in the names of C. J. Arroyo, N. J. Cogelia and R. J. Darsey. The core is enclosed in a thermal core wrap material, a corrugated metallic barrier and two helically wrapped translucent tapes. The foregoing sheath system, which depends on its reflection characteristics to keep heat away from the core, is well suited to larger size copper plenum cables. However, for smaller size cables such as optical fiber cables, the use of a heat reflective metallic shield is not only expensive, but is difficult to form about the core.
The prior art has addressed the problem of cable jackets that contribute to flame spread and smoke evolution also through the use of fluoropolymers. These, together with layers of other materials, have been used to control char development, jacket integrity and air permeability to minimize restrictions on choices of materials for insulation within the core. In one prior art small size plenum cable, disclosed in application Ser. No. 626,085 filed June 29, 1984 in the names of C. J. Arroyo et al and now U.S. Pat. No. 4,605,818, a sheath system includes a layer of a woven material which is impregnated with a fluorocarbon resin and which encloses a core. The woven layer has an air permeability which is sufficiently low to minimize gaseous flow through the woven layer and to delay heat transfer to the core. An outer jacket of an extrudable fluoropolymer material encloses the layer of woven material.
The use of fluoropolymers, with or without underlying protective layers, for optical fiber plenum cable jackets requires special consideration of material properties such as crystallinity, and coupling between the jacket and the optical fiber core detrimental effects on the optical fibers. If the jacket is coupled to the optical fiber core, the shrinkage of fluoropolymer plastic material, which is semi-crystalline, following extrusion puts the optical fiber in compression which results in microbending losses in the fiber. Further, its thermal expansion coefficients relative to glass are large, thereby compromising the stability of optical performance over varying thermal operation conditions.
What is still sought for optical fiber plenum cable is a relatively inexpensive sheath system having fire retardant and low smoke evolution properties. The sought-after cable desirably is as easy to manufacture as presently available products. Further, the sheath system should be one which minimizes and controls the coupling of the jacket material on the enclosed optical fiber core.